Field emitting devices are useful in a wide variety of applications, such as field emission flat panel displays, microwave power amplifiers, and nano-fabrication tools. See U.S. Pat. No. 6,283,812 by Jin, et al “Process for fabricating article comprising aligned truncated carbon nanotubes” issued on Sep. 4, 2001, and U.S. Pat. No. 6,297,592 by Goren, et al., “Microwave vacuum tube device employing grid-modulated cold cathode source having nanotube emitters” issued on Oct. 2, 2001. A typical field emitting device comprises a field emitting assembly composed of a cathode and one or more field emitter tips. The device also typically includes a grid closely spaced to the emitter tips and an anode spaced further from the cathode. Voltage induces emission of electrons from the tips, through the grid, toward the anode.
Small diameter nanowires, such as carbon nanotubes with a diameter on the order of 1-100 nanometers, have received considerable attention in recent years. See Liu et al., SCIENCE, Vol. 280, p. 1253 (1998); Ren et al., SCIENCE, Vol. 282, p. 1105 (1998); Li et al., SCIENCE, Vol. 274, p. 1701 (1996); J. Tans et al., NATURE, Vol. 36, p. 474 (1997); Fan et al., SCIENCE, Vol. 283, p. 512 (1999); Bower et als., Applied Physics Letters, Vol. 77, p. 830 (2000), and Applied Physics Letters, Vol. 77, p. 2767 (2000), Merkulov et al., Applied Physics Letters, Vol. 79, p. 1178 (2001); and Tsai et al., Applied Physics Letters, Vol. 81, p. 721 (2002); Teo et al., Nanotechnology, Vol. 14, p. 204 (2003). Such a structure with a nanoscale, high aspect ratio configuration is important for field emission applications because of the significant advantage of field concentration in such structures as the emitter operation can be conducted at a low applied voltage with much higher emission currents.
Long term reliability and stability of field emission emitter tips is of paramount importance. High-current, high-field operating conditions can subject emitter tips to Joule heating, oxidation, electromigration, and diffusion driven by the electrostatic stress near the sharp tip, all of which can lead to deterioration and even destruction of the emitters.
Instability of the emission current under certain emitter and vacuum conditions in carbon nanotubes is well known. It can, for example, be caused by the presence of oxygen impurity or other adsorbed gas species. See an article by K. Dean and B. R. Chalamala, J. Appl. Phy. 85, 3832 (1999). The oxidation rate will be generally proportional to the oxygen partial pressure. However, such undesirable oxidation is possible even in the ultra high vacuum conditions used for field emission devices. The variation of emission characteristics among different nanotubes (e.g. variation in nanotube height, tip sharpness, or size and shape of catalyst particles) can also cause significant instability problems as the strongly emitting nanotubes tend to deteriorate first. Some of the strongly emitting nanotubes can get very hot even in a display-type low current operations (e.g., >1600° C.). Continuous degradation of carbon nanotube tips can occur in the presence of cold cathode electric field and some unavoidable residual oxygen in field emission vacuum. The damage to nanotubes occurs through either a tip burning into CO2 or field evaporation of the tip under high current (and hence high temperature) operation.
Metallic Spindt tip emitters such as Mo or Ir tips also have emitter instability problems. For example, oxygen impurity in non-UHV vacuum conditions and ion bombardment and the occurrence of undesirable nanoprotrusions on metal emitter tips can result in a time-dependent increase in emission current and eventual catastrophic emitter failure.
Carbon nanotubes (CNT) are generally considered one of the best electron field emitters. Their high aspect-ratio geometry and resultant electric field concentration allows significant electron emission at relatively low applied fields. However, field emission is both a function of the field concentration factor and the work function of the emitter. Carbon nanotubes are not exceptionally good in the latter, having a relatively large work function (φ˜5.0 eV). There are many other materials which have lower work functions than CNTs, for example, ˜3.8 eV for TaC, ˜3.3 eV for TiN, ˜4.2 eV for Ta, and ˜4.5 eV for W. Some of these materials also are more stable (having strong atomic bonding and high melting temperatures).
One reason why these better materials have not been fully utilized for field emitters is the difficulty of fabricating them into an array of field-concentrating, sharp-tipped emitters. While a complicated lithography process enables fabrication of sharp Mo tips in Spindt emitters, they are complex and costly to fabricate and suffer reliability problems. The well known nanoprotrusion phenomenon and runaway emission, and sensitivity to oxygen have added to some serious barriers to successful, large-scale applications of such field emission cold cathodes. The carbides and nitrides have proven to be much more robust field emitters. See articles published by W. A. Mackie, T. Xie, M. R. Matthews, and P. R. Davis, in Materials Issues in Vacuum Microelectronics, Materials Research Society Symposium Proceedings Volume 509, p. 173 (1998), by A. A. Rouse, J. B. Bernhard, E. D. Sosa, D. E. Golden, Applied Physics Letters 76, 2583 (2000), and by H. Adachi, K. Fujii, S. Zaima, y. Shibata, Applied Physics Letters 43, 702 (1983). However, the construction of desirable field emitter configuration such as an array of spaced-apart nanotips, which is crucial for obtaining high emission current at low electric fields, has not been demonstrated for such carbide or nitride materials. Therefore, there is a need for nano array electron field emitters with improved field emission stability, at the same time with high current capability at low applied field.